Add ODIN_BUILD_FLAGS environment variable

[Feoramund]

This change appends each space-separated flag in the ODIN_BUILD_FLAGS environment variable to build-mode commands, allowing users to have default flags for the build/test/run actions.

Note that it will complain if you explicitly use a flag that is already in your environment variable, just as if you typed it. This doesn't do anything too magical.

If there's a need for an ODIN_CHECK_FLAGS, I can add that too.

[Kelimion]

I've been thinking on and off to add a -flags:<filename> option for the same purpose, so you can express your common flags in one go. Obviously the -flags option would be disallowed if encountered in a file, or this could go ad infinitum.

Let's see what @gingerBill makes of it.

[gingerBill]

It is a good question whether an environment variable or a file is better. I know many people want this but I do worry it'll make things a little more complicated.

[Feoramund]

I know many people want this but I do worry it'll make things a little more complicated.

I can see that happening now that I think about it, whether it's a file or an environment variable.

I'm okay with dropping this PR. Most shells I've used have some support for aliases, and failing that, people can put an executable script in their PATH as a shortcut that adds certain flags they use often.